1. Technical Field
The present invention relates to a fiber reinforced composite material which includes a thermoplastic resin as a matrix, and more particularly, a carbon fiber composite material which is excellent in development of reinforcing function by composite of carbon fibers.
2. Background Art
As a fiber-reinforced composite material using a carbon fiber, an aramid fiber, a glass fiber, or the like as a reinforcement fiber, an isotropic random mat is used from standpoints of formativeness and process simplicity. This random mat can be obtained by a spray-up method (“dry type” method) in which cut reinforcement fibers and a thermosetting resin are sprayed on a forming die simultaneously, or a method (“wet type” method) of adding a reinforcement fiber, which is previously cut, into a slurry containing a binder, and carrying out paper-making from the slurry.
It was known to increase fiber volume content (Vf) as means for improving mechanical physical property of a composite material, but it is difficult to increase the fiber volume content in a random mat using cut fibers for several reasons. For one, fibers are present along directions intersecting with the plane of the random mat at substantial angles and crossing of fibers is likely to occur. Also, it was difficult to adequately develop the strength of reinforcement fibers as compared with that from continuous fibers since fibers in the random mat are discontinuous. As a result, in known composite materials, the development rate of strength, which is a ratio of the tension strength of a shaped product and the tension strength of reinforcement fibers which constitute the shaped product, was generally less than about 5%.
Composites Part A, vol. 38 (2007), pp. 755-770 discloses a composite material of a random mat including carbon fibers and a thermosetting resin as a matrix, but the development rate of strength of the reinforcement fibers in the composite material is less than about 3.8%.
In recent years, techniques for improving mechanical physical property of a composite material using a random mat involved using chopped fiber bundles in which the cross-sectional areas are changed by cutting fiber bundles at slants of angles (JP-A-2009-114611 and JP-A-2009-114612). According to such a method, it was possible to improve the development rate of strength of reinforcement fibers from about 3 to 4% in the related art to about 7%, but not more than 10%.
As described above, a composite material in which a thermosetting resin is used as matrix was proposed. Usually, a fiber-reinforced composite material is obtained by heating and pressurizing a material called a prepreg which is previously obtained by impregnating a fiber-reinforced base material with a thermosetting resin, for 2 hours or more using an autoclave. In recent years, there was proposed an RTM (Resin Transfer Molding) method in which a thermosetting resin is poured into a mold in which a fiber-reinforced base material not impregnated with a resin is set, and the molding time was shortened substantially. However, even when the RTM method is utilized, it takes 10 minutes or more to mold a single part.
Therefore, a composite using a thermoplastic resin as a matrix instead of a thermosetting resin is attracting attention. However, a thermoplastic resin generally has a higher viscosity than a thermosetting resin, and thus the time for impregnating a fiber base material with the resin is long and, as a result, the tact time up to molding becomes long.
As a technique of solving the above matters, a technique called a thermoplastic stamping molding (TP-SMC: Thermoplastic-Sheet Molding Compound) is proposed (for example, Japanese Patent No. 4161409). This is a molding method of: heating chopped fibers, which are previously impregnated with a thermoplastic resin, at a temperature of more than the melting point; supplying the chopped fibers to a part in a mold and immediately after that, closing the mold; making fibers and a resin flowing within the mold to produce a product configuration; and cooling and molding of the product. In this technique, molding can be completed in short time of about 1 minute by using the fibers previously impregnated with the resin. This technique uses a molding material which is called SMC or a stampable sheet. In this thermoplastic stamping molding, due to flowing fibers and a resin within a mold, a thin product cannot be produced and the orientations of the fibers are not aligned and are difficult to control.
For a composite material including a thermoplastic resin as a matrix, there is proposed a technique of an injection molding of filament pellets including a reinforcement fiber (JP-A-9-286036). However, even for a filament pellet, the length of the pellet is limited, and that the reinforcement fiber is cut in the thermoplastic resin by kneading and mixing, thus the length of the reinforcement fiber is unable to be maintained. Further, reinforcement fibers are oriented in a forming method such as an injection molding, and thus an isotropic product cannot be obtained.